1. Field of the Invention
The present invention relates generally to a recording head for recording or printing images such as characters, figures, or other patterns, by applying an electric current to a ribbon or film, or to a recording medium. More particularly, the invention is concerned with a structure of recording electrodes used for such an electrothermal recording head, electrical discharge recording head or electrostatic recording head.
2. Discussion of the Prior Art
Various structures or arrangements of recording electrodes have been proposed for recording heads, for applying an electric current to effect a recording operation, by using a recording medium of electrical discharge destruction type, a recording medium having heat-sensitive color former, or a recording medium such as a thermosensitive paper, or by using a ribbon or film which has at least an electrically resistive layer. Examples of such recording heads are disclosed in laid-open Publication Nos. 61-171349, 61-272172, 60-78772, 63-51152, 63-87264, and 61-23,0966 of unexamined Japanese Patent Applications. These recording heads are operated such that a plurality of recording electrodes are held in contact with a ribbon or film to apply electric currents to local portions of its electrically resistive layer, so as to produce heat for printing on a recording medium, or such that the electrodes are held in contact with a recording medium having a vapor-deposited aluminum layer, a black-colored layer and a base layer, to remove local portions of the aluminum layer by electric discharge destruction, so as to expose the corresponding local portions of the black-colored layer.
Further, laid-open Publication Nos. 58-12790, 58-104787, 61-37493, 63-3027,9 and 63-160855 of unexamined Japanese Patent Applications disclose recording heads for applying an electric current to an ink layer or a heat-sensitive layer which is formed on the surface of a sheet, ribbon, web or other film-like member, or which forms an inner layer of the film-like member. The ink layer may be an electrically conductive layer, an electrically resistive layer or electrochemical reaction layer. The heat-sensitive layer may contain an electrolyte and produces a color due to exposure to heat.
The recording electrodes used for such recording heads must be formed with high accuracy, to assure high degrees of uniformity and reproducibility of images printed in a matrix of dots. For improved resolution of the printed images, the recording head must have a high density of electrodes, namely, a sufficiently large number of electrodes per unit distance in the direction in which the electrodes are arranged in mutually spaced-apart relation. For increased recording speed and enhanced recording quality, the electrodes must have a sufficiently large thickness, and the spacing between the adjacent electrodes must be sufficiently short.
Laid-open Publication No. 61-171349 discloses a method of preparing an array of recording electrodes, in which electrically conductive wires or needles are arranged and fixed on a suitable support member. However, this method does not permit the wires or needles to be suitably disposed with high efficiency. Where the required number of electrodes is relatively large, the method is substantially impracticable from the standpoints of production technique and economy. Further, the method does not permit the electrodes to be arranged in spaced-apart relation at a pitch small enough to assure desired quality of printed images.
There are also known methods which utilize photolithography to form the recording electrodes. In this method, an electrically conductive layer is initially formed on an electrically insulating substrate, in a suitable process, such as thin-film forming method by plating or sputtering on the substrate, thick-film forming method by applying a coating of a suitable paste or resinate to the substrate, or bonding of an electrically conductive sheet to the substrate. The formed electrically conductive layer is photolithographically patterned to produce a desired array of the recording electrodes. These methods permit a comparatively large number of the electrodes to be easily formed with relatively high density and accuracy, and are therefore widely practiced in the art. However, the lithographic methods do not permit the electrically conductive layer or the electrodes to have a thickness which is larger than the spacing between the adjacent electrodes. Thus, the recording heads using the electrodes prepared by the methods in question are not satisfactory in terms of the recording speed and quality of printed images.
Also known is a method which uses a press forming machine, to pierce an electrically conductive sheet such that apertures or cutouts are formed through the sheet so as to define suitably arranged electrodes. This method is limited in the minimum width of the apertures, and the thickness of the electrodes (thickness of the sheet) cannot be greater than the spacing of the electrodes. The recording heads using the electrodes prepared according to this method are not satisfactory in terms of the recording speed and quality of printed images.